1. Technical Field
This invention relates to the general field of electron beam lithography and, more particularly, to real-time correction of the lithography process to compensate for heating of the resist by thermal diffusion from areas previously written, and most particularly to methods and apparatus for calculating resist heating in real-time, sufficiently rapidly to permit correction of the lithography process.
2. Description of Related Art
The production of precise patterns on surfaces is a necessary stage in the fabrication of integrated circuits, and finds applicability in many other commercial environments as well. The typical method for creating such patterns is to coat the surface to be patterned with a chemical that undergoes a chemical transformation upon exposure to energy, a xe2x80x9cresist.xe2x80x9d Positive resists undergo chemical transformation on exposure to energy leading to removal of resist from the surface to be etched in the regions so exposed. Negative resists undergo other chemical transformations, such as cross-linking, leading to removal of resist in regions not exposed to energy. Both positive and negative resists are commercially useful. Thus, exposing a resist-coated surface to the appropriate pattern of energy leads to selective removal of resist according to that pattern (exposed or masked), uncovering selected regions of the underlying surface to further chemical etching in a subsequent etching step. Removal of all resist following surface etching leads to the desired pattern etched into the surface.
The energy incident on the resist is typically either electromagnetic or a beam of particles, typically ions or electrons (xe2x80x9ce-beamxe2x80x9d). In addition, the energy may be directed onto the resist in one of two general ways: 1) through a mask having both transparent and opaque regions therein permitting selective passage of the incident energy to create the desired pattern of exposure on the underlying resist, or 2) as a focused beam, guided so as to impact selectively only those areas requiring exposure. Exposure through a mask is the presently preferred technique for producing numerous identical patterns at reduced costs. However, the mask itself must first be made, most commonly by focused beam impact. Thus, focused beam exposure of resists remains a necessary step in the production of masks for lithography.
Direct beam xe2x80x9cwritingxe2x80x9d of patterns onto resists has several advantages over use of a mask. Among these are avoiding the complications of alignment and registration of the mask and more precise patterning accomplished by precisely focused beams. Thus, beam lithography finds applicability in many areas of technology in addition to mask creation. However, the discussion herein will be particularly directed to e-beam lithography for the production of masks, although other applications for the methods described herein will be apparent to those having ordinary skills in the art. For economy of language we will describe e-beam lithography as typically used in the manufacture of masks, not intending thereby to limit the scope of the invention merely to electrons.
Precise patterning requires precise exposure of resist. For concretness of our description, we will consider the case of positive resists, which are removed from the underlying layer for subsequent etching where the positive resist is exposed to the incident e-beam. Completely analogous effects are present for negative resists as well understood in the art. A sharp boundary is desired between exposed regions and unexposed regions for both types of resist, permitting the mask designer to use more densely packed components without interference and overlap of imprecisely exposed adjacent patterns.
Precise exposure of resist requires a detailed understanding of the sensitivity of the resist to e-beam exposure. The exposure of resist to an e-beam, called the dose, is typically measured in microcoulombs per square centimeter (xcexcC/cm2). The sensitivity of the resist means the electron dose (in xcexcC/cm2) necessary to create the desired pattern in the resist upon development. This sensitivity is a function of the resist composition, the energy of the incident electron beam, the temperature of the resist, the resist development process and other factors as well. The changes of resist sensitivity with its temperature at the time writing occurs is a particular concern of the present invention.
Scanning the entire surface to be exposed typically involves scanning of the electron beam over a limited area while mechanical translating the surface over a much larger area. Electron beam scanning typically involves rapid transitional motion of the beam across the surface (up to approximately 10,000 cm/sec), but only covering a small region of the surface (typically around a millimeter in lateral extent). The entire substrate is moved mechanically at about 1 cm/sec but over an extent of lateral traverse sufficient that the e-beam exposes the entire surface.
It is helpful to emphasize that exposure of the resist by e-beam impact and heating of the resist are two conceptually distinct phenomena. The chemical activity of the resist leading to its useful lithographic properties is initiated by e-beam impact. The efficacy of electrons in causing this chemical activity is defined as the sensitivity of the resist. The sensitivity of the resist to e-beam impact depends in turn on many factors including the temperature of the resist at the time it is exposed. Thus, changing the temperature of the resist changes its sensitivity which may require changing the dose of electrons in order to achieve proper exposure. Failing to take into account changes in resist sensitivity with temperature may lead to overexposure of the resist, exposure of the resist in regions not intended to be fully exposed, and less precise patterns. Pattern xe2x80x9cbloomingxe2x80x9d is the undesired result.
Heating of the resist occurs in two ways: 1) As an inherent adjunct effect to the impact by electrons intentionally directed onto the resist for exposure. This heating is always present in e-beam lithography and is taken into account when the resist is calibrated to specify the correct exposure. 2) In high voltage lithography, most of the electron beam energy passes through the resist and the underlying mask layer (typically very thin) and penetrates the substrate where most of the energy is deposited as heat. (An exception occurs when thin substrates are used, typically in the manufacture of X-ray masks, where the substrate is itself a film so thin that most of the beam energy passes through it). Electron diffusion in a thick substrate deposits the heat from a single e-beam flash in the substrate, typically in a volume 10 or more microns (micrometers) in lateral extent (perpendicular to the e-beam direction). Subsequent thermal conduction transports a portion of this heat to the substrate surface where it heats the resist in a zone that may be tens of microns in lateral extent a few microseconds following the flash, increasing to a millimeter across after several milliseconds. (Exact numbers will depend on beam energy, the composition of the substrate and its thermal properties). Thereafter the heat has diffused so much as to have no significant effect on resist exposure. It is this second type of heating that this invention addresses and denotes as xe2x80x9cproximity heatingxe2x80x9d or xe2x80x9cresist heating.xe2x80x9d Such resist heating depends on the previously written pattern and the time history of the pattern writing. This variability makes resist heating particularly challenging to estimate in designing a process for high accuracy e-beam writing.
xe2x80x9cProximity heatingxe2x80x9d or xe2x80x9cresist heatingxe2x80x9d as used herein is not to be confused with the xe2x80x9cproximity effectxe2x80x9d related to the chemical effects of scattered electrons in the resist. Electrons in a beam passing through matter will from time to time encounter atomic nuclei or orbital electrons and undergo deflection from their line of travel, with or without loss of energy in the deflecting collision. The xe2x80x9cproximity effectxe2x80x9d relates to the chemical effect these scattered electrons have exposing the resist, perhaps relatively far from the intended exposure zone at which the e-beam is directed. Scattered electrons within the resist may lead to exposure away from the desired exposure zone. Backscattered electrons from layers below the resist may re-enter the resist and also produce deleterious exposure. Many approaches have been suggested to ameliorate the effects of these scattered electrons as they lead to unwanted exposure of the resist. Bohlen et. al. (U.S. Pat. Nos. 4,426,584 and 4,504,558) suggest a second exposure to the incident e-beam designed to correct for dosage losses or (for e-beam exposure through a mask) the use of two complimentary masks. Several ways to correct for the electron beam dosage have been suggested, including the work of Watson (U.S. Pat. No. 5,736,281), Ashton et. al. (U.S. Pat. No. 5,051,598), Owen et. al. (U.S. Pat. 5,254,438), and Chung et. al. ( U.S. Pat. No. 5,432,714). In all cases, however, the focus of this work is to prevent or reduce the chemical effect of scattered electrons in causing undesired exposure of the resist. In contrast, the present invention relates to the thermal effect of both incident electrons and scattered electrons as they heat the target indirectly by conduction of heat deposited elsewhere, and the changes in resist sensitivity caused by this heating.
Calculations of resist heating are typically based upon a numerical solution of the appropriate diffusion (partial differential) equations. Heat sources caused by electron impact may be represented by analytic approximations, or derived directly by numerical Monte Carlo simulation of the electrons as they penetrate into targets, including resists, depositing their energy therein. Many prior methods have proven in practice to be too slow in comparison with the speed of e-beam writing to allow real-time computation of resist heating and adjustment of the writing process in response. The present invention provides methods for rapidly predicting resist heating on a time scale comparable with the e-beam writing speeds. This real-time prediction of resist heating allows the properties of the e-beam and/or the writing process to be adjusted while writing is underway to compensate for resist heating.
Resist heating has been the subject of several calculations and measurements. Ralph et. al. describe methods for computing resist heating by numerical integration of diffusion equations in xe2x80x9cProceedings of the Symposium on Electron and Ion Beam Science and Technology, Tenth International Conferencexe2x80x9d, p. 219-2330 (1983). Babin et. al. also describe methods for the numerical simulation of resist heating and the comparison of such calculations with measured values. SPIE, Vol. 3048, p. 368-373 (1997) and J. Vac Sci Technol. B Vol. 16, pp. 3241-3247 (1998). Additional calculations of resist heating and comparison with measured values have been reported by Yasuda et. al. in and J. Vac Sci Technol. B Vol. 12, pp. 1362-1366 (1994).
Several approaches have been taken to calculate and/or correct for resist heating and to formulate methods for performing such calculations in real-time. These include the work of Veneklasen et. al. (U.S. Pat. No. 5,847,959 and the pending patent application of Innes et. al. (09/343,960, commonly assigned with the present application). The present invention is an alternative method for estimating and correcting for resist heating that may be implemented on a general digital purpose computer, Field-Programmable-Gate-Array, Digital Signal Processor, etc. Very rapid calculations of resist heating result. Dedicated analogue hardware is also described that permits the resist heating of the resist to be calculated according to the procedures of the present invention very rapidly in real-time, allowing for the correction of the patterning process also in real-time.
Precise writing of patterns in a resist requires precise exposure of the resist which, in turn, requires precise knowledge of the sensitivity of the resist to e-beam impact. Resist sensitivity depends upon the temperature of the resist at the time of writing. Thus, the present invention relates to methods and procedures for determining resist temperature during processing and adjusting process parameters, including reducing the beam current, to compensate for increased resist sensitivity. Typically, the resist temperature rise predicted by the present invention for the point of writing will be multiplied by a factor relating the temperature sensitivity of the resist. The result is a correction applied to the beam current (or dwell time) to provide more accurate resist exposure. The correction will typically be a multiplicative factor less than 1 (for most positive resists) by which the beam current is to be adjusted to correct for resist heating at the point of writing. In an analogous manner, corrections to beam dwell time may be used alternatively or in addition to beam current corrections. It is envisioned that e-beam current or the dwell time of each spot or xe2x80x9cflashxe2x80x9d may be adjusted. Pattern blooming is thereby reduced.
The present invention relates to methods of predicting resist heating in real-time as the writing proceeds thereby enabling beam compensation to be performed in real-time. Methods of achieving high processing efficiency are described.
A linear superposition approximation is described that provides a procedure for estimating the resist temperature at the point presently being written from previously written points. These techniques are most effective in the case of raster or serpentine scanning in which the e-beam scans over every line in sequence, creating the desired pattern in the resist by depositing the correct amount of energy (including zero energy) at each point (pixel) in turn. Thus, for raster or serpentine scanning there is a definite and known relationship between the time of writing each pixel and that pixel""s location on the surface. By xe2x80x9cwritingxe2x80x9d we mean directing the e-beam towards a particular pixel on the surface, whether or not any energy is deposited. This is in contrast to xe2x80x9cvector scanningxe2x80x9d in which the e-beam is directed to certain regions requiring non-zero energy exposure and skips other regions entirely. Prediction of resist heating for vector scanning is more complex due to this unknown relationship between the time and location of writing pixel i and the time and location of writing pixel j. Knowing i, j and the properties of the equipment allows one to know precisely for raster or serpentine machines the time and distance relationship between any two pixels, i,j.
The present invention makes use of the thermal history of the pattern previously written as that history is recorded in the line of pixels immediately preceding the line whose temperature is to be determined prior to e-beam exposure, and a single number representing the thermal history of lines written before the immediately preceding line. That is, the temperature of line 2 (the second line written) is determined from a knowledge of the thermal history of line 1 (the first line written) and the time and distance dissipation of heat at the location and time of writing line 2. Resist temperatures at the time and location of writing line 3 are determined only from knowledge of the thermal history of line 2, and a single taking into account thermal effects arising from previously written lines. Therefore, in this approximation, the temperature of the resist at the time and location of writing line i is determined from the properties of the immediately preceding line, line (ixe2x88x921) and a single numerical parameter. The thermal effects of lines (ixe2x88x922), (ixe2x88x923) . . . 1 on line i have been iteratively accumulated in the properties of line (ixe2x88x921) and in the parameter The iterative methods for calculating resist heating in the present invention may be implemented digitally on a variety of devices. In addition, specific analogue circuitry to reproduce this iterative method is described, leading to rapid calculations of resist heating. Advantages of the present invention include the prevention or mitigation of pattern blooming as incident electrons heat the resist and broaden the region of exposure.